This invention relates to a neutron detector and more particularly to a solid state semiconductor neutron detector formed from pyrolytic boron nitride and to a method of manufacture of a pyrolytic boron nitride neutron detector.
Neutrons are uncharged elemental particles which do not ionize matter as they pass through it. Accordingly, neutron particles are difficult to detect. Thermal neutrons are produced by splitting atoms such as Uranium 235 in a nuclear reactor and slowing the velocity of the fissioned neutrons through collisions with some moderating material. A Geiger counter is a conventional detector for detecting neutrons. The Geiger counter is a gas filled tube which may be filled with 3He or with BF3 but has limited utility since it is both bulky and expensive to manufacture. Moreover, the Geiger counter requires repeated calibration. Other detection devices which are used to detect neutrons are based on the principle of scintillation which is an indirect process in which the interaction of neutrons with a detector scintillation material generates light which, in turn, permits light detectors to be used from which the level of neutron presence can be established. However, the light detectors need to be sensitive to the wavelength of the light. Otherwise, an emulsion film must be used. The use of scintillation detectors for detecting neutrons is inefficient because optics cannot gather all of the light and some of the light is reabsorbed by the scintillating material. Furthermore, light detectors have an inherent sensitivity limit to all wavelengths.
At present no solid state detector is commercially in existence capable of detecting thermal neutrons. The present invention utilizes pyrolytic boron nitride to form a solid state thermal neutron detector in which a direct electrical signal is formed proportional to the alpha particles generated from the interaction of the colliding neutrons with the boron-10 isotope in pyrolytic boron nitride. Pyrolytic boron nitride (hereinafter xe2x80x9cpBNxe2x80x9d) is formed by chemical vapor deposition using a process described in U.S. Pat. No. 3,182,006, the disclosure of which is herein incorporated by reference, involving introducing vapors of ammonia and a gaseous boron halide such as boron trichloride (BCL3) in a suitable ratio into a heated furnace reactor to deposit boron nitride on the surface of an appropriate substrate such as graphite. The boron nitride is deposited in layers and when separated from the substrate forms a free standing structure of pBN.
Pyrolytic boron nitride (xe2x80x9cpBNxe2x80x9d) is anisotropic and has a hexagonal crystal lattice. In fact, most boron nitride made of CVD is composed of hexagonal crytallites in which the a- and b-axes are predominantly oriented parallel to the deposition surface. The hexagonal structure and preferred orientation impart highly anisotropic properties to the pBN. Because of symmetry, the a- and b-axes are equivalent, so it is convenient to describe pBN as having only two sets of properties, i.e., in the ab direction and in the c direction. In a single crystal of BN, the xe2x80x98ab planesxe2x80x99 are perpendicular to the layers. In pBN, the xe2x80x98ab planesxe2x80x99 have no preferred orientation except in the direction normal to the deposition layers. The crystographic planes, such as the c plane, are normal to their axes, so that the c plane in pBN is predominantly parallel to the deposition layers. Since the pBN deposits are for practical purposes limited to a few mm thick, the edge surface area is small in comparison with that attainable on the deposition surface. However, all previous attempts to capture neutrons using a pBN detector fabricated in a conventional fashion and oriented to collect neutrons through the deposition layers, i.e., the predominantly c-axis direction, have yielded poor results.
It has been discovered in accordance with the present invention that by forming a pBN detector with electrical contacts parallel to the c-axis direction, i.e. normal to the ab planes, the sensitivity to thermal neutrons is significantly increased. Moreover, in accordance with the present invention it is preferred that the pBN material is formed with a predetermined thickness of between one micron to one mm and that electrical contacts are applied the pBN material on either side of this thickness.
A solid state neutron detector of pBN may be formed in accordance with the present invention in which a direct electrical signal is generated from alpha particles produced in response to the interaction of neutrons with the pBN detector material by applying electrical contacts on two ends of the pBN detector material aligned parallel with the xe2x80x9cab planesxe2x80x9d and by connecting the electrical contacts to an electrical analyzer.
The pBN neutron detector of the present invention comprises a multi-layered pBN material with a crystalline lattice structure having two opposed edge surfaces aligned to correspond with the xe2x80x9cab-planesxe2x80x9d of the structure, metallized contacts contacting each of said opposed surfaces for conducting electrons to detect neutrons and with the structure having a thickness between the opposed edge surfaces of between one micron and one mm. The preferred thickness for pBN is 1.00 microns (0.004 inches). An array of metallized contacts may be formed in the pBN material as layered strips parallel to one another and separated by a distance of between 25 and 100 microns with a preferred separation of 50 microns. The array of metallized contacts should be placed on a pBN surface aligned in a direction parallel to either the a plane or b plane respectively.
The method of forming a neutron detector in accordance with the present invention comprises the steps of depositing multiple layers of pBN having a crystalline lattice structure with its crystallographic xe2x80x98c planexe2x80x99 predominantly parallel to the deposited layers to form a given geometry with two opposite sides aligned parallel to the xe2x80x98ab planesxe2x80x99 of the structure and a thickness of between one micron and one mm, applying metallized contacts to said opposite sides, and orienting said detector relative to a source of neutrons such that the neutrons will pass through the volume of the detector and cause electrons in response to alpha particles generated from the presence of of neutrons to conduct through the structure parallel to the c plane.